laser tripwire and alarm using snap circuits

Laser Tripwire and Alarm Using

Snap CircuitsWritten By: KRA5H

PARTS:

Base Grid (11” x 7.7”) # 6SC BG (2)

100K ohm Resistor # 6SC R5 (2)

1 0.02uF Capacitor # 6SC C1 (1)

1 Speaker # 6SC SP (1)

1 Battery Holder (2-AA) # 6SC B1 (3)

1 PNP Transistor # 6SC Q1 (1)

1 NPN Transistor # 6SC Q2 (2)

SCR # 6SC Q3 (1)

Pivot Stand Base # 6SC PSB (1)

Pivot Post # 6SC PSP (1)

Pivot Top # 6SC PST (1)

Photosensitive Resistor # 6SC RP (1)

Relay # 6SC S3 (1)

Single Snap Conductor # 6SC 01 (5)

Conductor with 2-snaps # 6SC 02 (6)


Laser Tripwire and Alarm Using Snap Circuits

© Make Projects www.makeprojects.com of 15

http://makeprojects.com/Item/Base_Grid_%2811%E2%80%9D_x_7.7%E2%80%9D%29_6SC_BG

http://makeprojects.com/Item/100K_ohm_Resistor_6SC_R5

http://makeprojects.com/Item/1_0.02uF_Capacitor_6SC_C1

http://makeprojects.com/Item/1_Speaker_6SC_SP

http://makeprojects.com/Item/1_Battery_Holder_%282-AA%29_6SC_B1

http://makeprojects.com/Item/1_PNP_Transistor_6SC_Q1

http://makeprojects.com/Item/1_NPN_Transistor_6SC_Q2

http://makeprojects.com/Item/SCR_6SC_Q3

http://makeprojects.com/Item/Pivot_Stand_Base_6SC_PSB

http://makeprojects.com/Item/Pivot_Post_6SC_PSP

http://makeprojects.com/Item/Pivot_Top_6SC_PST

http://makeprojects.com/Item/Photosensitive_Resistor_6SC_RP

http://makeprojects.com/Item/Relay_6SC_S3

http://makeprojects.com/Item/Single_Snap_Conductor_6SC_01

http://makeprojects.com/Item/Conductor_with_2-snaps_6SC_02





Jumper Wire 18" (Black) # 6SC J1 (1)

Jumper Wire 18" (Red) # 6SC J2 (1)

Jumper Wire 8" (White) # 6SC J3F (1)

Jumper Wire 4" (Blue) # 6SC J4 (1)

Laser pointer (1)RadioShack.com has a green laser pointer similar to the one I use in this article. If youhappen to have the Spynet Laser Tripwire laser you may use it as your laser trip wire. Iused it in the demonstration video. It has two axes of rotation, horizontal and vertical,and is much easier to aim at the photoresistor.

Magnetic Chip Clip (1)Serves the dual purpose of clamping the on/off switch in the "on" position and as a standfor the laser pointer--I got mine for free from an Indiana Fever game.

Object(s) of your choosing to stack under the laser pointer to aim it at the photoresistor(1)In this article I used a couple of packs of peanutbutter crackers.

SUMMARY

In this article you will learn how to build a laser tripwire and alarm out of Snap Circuits. Youwill build two circuits: the laser tripwire circuit and the alarm circuit. The alarm is a verysimple two-transistor oscillator that is switched on by the relay in the laser tripwire circuit.Once you have built the two circuits, you will then learn how to set up the laser pointerwhere somone is likely to break the laser beam and trip the alarm. The Spynet LaserTripwire was the inspiration for this build. I wanted to see if I could build my own lasertripwire. I also improved it by adding the relay to switch on an external circuit. I used thetwo-transistor oscillator as an example, but one could use the relay to switch on any numberof external circuits such as floodlights, home alarm systems, etc.

Actually, the laser tripwire was an interesting problem to try to solve. In a standardtransistor-SCR (thyristor) circuit, you often see an LED where I put the relay.






http://makeprojects.com/Item/Jumper_Wire_18%22_%28Black%29_6SC_J1

http://makeprojects.com/Item/Jumper_Wire_18%22_%28Red%29_6SC_J2

http://makeprojects.com/Item/Jumper_Wire_8%22_%28White%29_6SC_J3F

http://makeprojects.com/Item/Jumper_Wire_4%22_%28Blue%29_6SC_J4

http://makeprojects.com/Item/Laser_pointer

http://makeprojects.com/Item/Magnetic_Chip_Clip

http://makeprojects.com/Item/Object%28s%29_of_your_choosing_to_stack_under_the_laser_pointer_to_aim_it_at_the_photoresistor

The SCR is advertised as a latch-on device--once switched on, it is supposed to stay on, butit don't work as advertised:

If you replace the relay with an LED and break the laser beam (or other light source), that is,stop the light source from shining on the LDR (light dependent resistor) the LED switches onas advertised. When you restore the light shining on the LDR, however, the LED switchesback off again.

So, I decided to insert the relay to switch on another circuit that would stay on even whenpower was cut to the relay. Oddly enough, the relay somehow keeps the SCR switched onbut I'm not sure why.

In a single stroke (and by sheer accident!) I solved the problem and can now switch on anyexternal circuit via the relay without having to add any additional components to make theexternlal circuit remain on. And it will remain on until I switch the power off to the lasertripwire circuit.

Wierd. But it may be useful to others working on any number of "electric eye" applications

All Snap Circuits parts used here come from two sets: Snap Circuits Extreme 750, and SnapCircuits Green. Both sets are available at your local RadioShack store. Individual parts canbe ordered from http://cs-sales.net/sncirepa.html if needed



http://cs-sales.net/sncirepa.html

Step 1 — Overview

Snap Circuits is an educational toythat teaches electronics withsolderless snap-together electroniccomponents. Each component hasthe schematic symbol and a labelprinted on its plastic case that iscolor coded for easy identification.They snap together with ordinaryclothing snaps.

The components also snap onto a10 X 7 plastic base grid analogousto a solderless breadboard.

All the kits include manuals printedin color with easy-to-followdiagrams to assemble the projects.The illustrations for each projectlook almost exactly like what thecomponents will look like on thebase grid when finished.

Because the electronic symbol isprinted on each electroniccomponent, once the project iscompleted, it will look almostexactly like an electronicschematic.



Step 2 — Assembling the alarm circuit.

The alarm circuit is a very simple two-transistor oscillator circuit.



Step 3

When the laser tripwire relaycloses the alarm circuit (switchesthe alarm circuit on), here's whathappens:

Step 1. The 0.02uF capacitor C1begins to charge up. Once thecapacitor has enough charge, it willswitch transistor Q2 on.

Step 2. When Q2 switches on, thiscauses transistor Q1 to switch on.

Step 3. When Q1 switches on, itsupplies power to the speakerwhich makes a sound.

Step 4. Capacitor C1 begins todischarge to ground.

Step 5. Once the charge in thecapacitior is low enough, thisswitches transistor Q2 off.

Step 6. When Q2 switches off, thiswill switch off transistor Q1. WhenQ1 switches off this cuts the powerto the speaker and the speakerstops making a sound

Step 7. Go back to step 1. Thisprocess repeats over and overagain, ad infinitum, and you hear ahigh pitched tone until you cut thepower to the circuit.



Step 4 — Laser Tripwire part 1.

The photoresistor acts as a darkdetector. As long as the laser beamshines on the photoresistor, thisreduces its resistance. Sinceresistance in this part of the circuitis reduced, current can flow to thebase of the transistor Q2 switchingit on.

While Q2 is switched on, currentwill not flow to the SCR Q3. Oncethe laser beam is cut, that is,blocked from shining on thephotoresistor, the photo resistorwill increase its resistance andcurrent will be cut from the base ofthe transistor Q2. When Q2 isswitched off, current can then flowto the SCR Q3 switching it on.

When Q3 is switched on, this willpower the electromagnet of therelay.




You’ve probably seen a relay racewhere one runner hands off a batonto another runner. Similarly, anelectronic relay hands off controlfrom one circuit to another. A relayis a very simple device consistingof an electromagnet, an armature(a switch that closes whenattracted by the electromagnet),and a spring that is connected tothe armature.


When the SCR Q3 is switched on,current flows to the relay chargingup the electromagnet. Theelectromagnet creates a magneticfield that attracts the armature toclose the alarm circuit (switchesthe alarm circuit on), and you hearthe high pitched tone created bythe alarm circuit.




What's interesting about the SCR isthat once it is switched on, itremains on providing current to therelay and the alarm sound willremain on until you turn the slideswitch S1 off.




After a person or object passesthrough the laser beam, the beamwill shine on the photoresistoragain reducing the resistance.Normally this would restore thecurrent to the base of transistorQ2.

This would then switch Q2 onagain, cutting the power to the SCRQ3 switching it off and thus cuttingthe current to the relay'selectromagnet switching the alarmcircuit off.

But, in the case of this circuit, therelay seems to be drawing enoughpower from the batteries to keepthe SCR switched on andpreventing the transistor fromswitching on again and cutting thepower to the SCR.



Step 9



In the case of the green laserpointer, you have to hold the on/offbutton down to fire the laser andwhen you release the button, thelaser switches off. So, you have tohave a method of holding the laserpointer switch in the "on" position.

One easy method is to take a twisttie from a loaf of bread and wrap itaround the laser pointer and theon/off button and then twist the tietightly so that it holds the switch inthe "on" position. I, on the otherhand, use a different method tokeep my green laser pointerswitched on.

I, serendipitously, found that theIndiana Fever magnetic chip clipopens wide enough to insert mygreen laser pointer in its jaws. Thechip clip also has a set of sawtoothteeth that catch on the barrel of thelaser pointer holding it in positionand preventing the clip fromslipping off the barrel.

Also the chip clip clamps on theon/off switch and is strong enoughto hold it in the "on" position. Theclip can also be used as a stand toraise the laser pointer off theground. You'll have to stacksomething underneath the laserpointer to aim it at thephotoresistor--in my case I used acouple of packs of peanutbuttercrackers.



crackers.

No worries if you don't live inIndianapolis nor attend professionalor minor league sports events. Youcan probably find one of these chipclips being given away free with theappropriate corporate logo stampedon them at conventions, Earth Daycelebrations, county/state fairs andso on.



Step 10



This document was last generated on 2012-11-01 12:12:09 AM.

In this picture I used the SpynetLaser Tripwire laser. It is mucheasier to aim at the photoresistorsince it has two axes of rotation--vertical and horizontal.

To set the laser tripwire ensure thatthe laser beam is set across aspace where someone is likely tobreak the beam such as across adoorway or across a hall.

Recall too that that the laser dotfrom the pointer will project a fairlylong distance so you could bounceit off a set of mirrors around theentire perimeter of your home sothat anyone walking on to yourproperty will break the beam.

For example, if your neighbor downthe street lets his or her dog do itsbusiness on your lawn withoutcleaning up after it, you can catchhim or her in the act.

Once you have the laser dotpositioned on the photoresistor,turn the circuit on by moving theslide switch S1 to the "on" position.Pass your hand through the laserbeam to test it. You will hear therelay click and the alarm sound.Switch the circuit off and back onagain to reset it.

You can watch a video of the LaserTripwire being demonstrated here.



http://vimeo.com/50846252

laser tripwire and alarm using snap circuits

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